Feedstock Technology for Reactive Metal Injection Molding: Process, Design, and Application

By Peng Cao and Muhammad Dilawer Hayat

Feedstock Technology for Reactive Metal Injection Molding: Process, Design, and Application provides an authoritative guide on the basics of feedstock technology and the latest developments in binders for titanium metal injection moulding and their potential implications. In addition, the book presents challenges that MIM technology of reactive metals is currently facing and potential solutions for commercial success. As both commercial growth and research development are fundamentally driven by the economics of manufacture, this book presents the problems and potential solutions regarding reactive metals, making it a valuable resource for engineers intending to utilize MIM in commercial product design.

• Provides comprehensive details and case studies on the feedstocks currently under extensive development, in research and in the commercial domain
• Discusses the most recent developments of binder chemistry and design, along with the most critical challenges in MIM technology
• Includes comprehensive evaluations with regard to feedstock characterization and impurity control

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 16, 2020
• ISBN: 9780128175026

Peng Cao

Dr. Peng Cao is an associate professor of Materials Engineering in the Department of Chemical and Materials Engineering, the University of Auckland, New Zealand. His group focuses on developing titanium powder technologies and new energy-storage materials. He has published over 200 journal articles, and five edited/authored book. He has organized 10 international conferences, of which he chaired or co-chaired three, and delivered approximately 20 keynotes/invited talks.

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Chapter 1

Reactive powder metal injection molding

Abstract

This chapter briefly introduces each step involved in metal injection molding. It also introduces the evolution of metal injection molding technologies, opportunities, applications, and some constraints.

Keywords

Metal injection molding; applications; MIM products

1.1 Metal injection molding—a standout manufacturing technology?

The market of metal injection molding (MIM) has expanded significantly over the last decades to include a broad array of applications such as consumer electronics, automotive, medical, and firearms. MIM fabrication results from the application of plastic injection molding technology to powder metallurgy. It is a low-cost forming method best suited for the metals and alloys that are difficult to machine or cast. The process is used to make small-to-medium and complex-shaped parts from metal or alloy powders and relies on shaping metal particles and subsequently sintering those particles. Hence, the process is capable of producing parts having higher strength compared with die casting, improved tolerances compared with investment or sand casting, and more shape complexity compared with most other forming routes. The competitive advantage comes from the ability of MIM to manufacture complex parts at high production rates and near-net-shaping capability, which results in high yield savings. Conventional production processes are being increasingly replaced by MIM—machining, casting, and press-sintering are some of the processes that have been replaced by MIM.

1.2 Overview of metal injection molding

1.2.1 Metal injection molding processes

Although powder injection molding (PIM) was first demonstrated during the 1930s when the ceramic spark plug bodies were produced, MIM did not achieve commercial status until the 1970s. The delay was due to the lack of sophistication in the process equipment. With the advent of microprocessor-controlled processing equipment, such as molders and sintering furnaces, which enabled repeatable and defect-free cycles with tight tolerances, the manufacturing infrastructure of the MIM strategy improved dramatically.¹

MIM attracted major attention in 1979 when two design awards were won. One award was for a screw seal used on a Boeing jetliner. The second award was for a niobium-alloy thrust chamber and injection for a liquid-propellant rocket engine.² These awards provided the necessary springboard for extensive research in this field. Several patents emerged, with one of the most useful being issued in 1980 to Ray Wiech. From this beginning, a host of other patents, applications, and firms strictly dealing with MIM rose. By the middle 1980s, the MIM technology had developed a firm base in the manufacturing sector. Since the mid-1990s, the MIM technology has expanded to include an array of different material families and new and innovative product designs that were not possible with conventional processing.

The key steps of MIM include (1) selecting and tailoring a powder for the process; (2) mixing metallic powder and a binder system to form a homogenous feedstock; (3) molding the feedstock to achieve the required shape and geometry (green parts); (4) removal of the binder while keeping the geometry (debinding); (5) sintering the debound parts (brown parts) to achieve the desired mechanical properties; and (6) post sintering treatments to further improve properties if required. The steps involved in the MIM are illustrated in Fig. 1.1.

Figure 1.1 Steps involved in a MIM operation.³

MIM can process any metal if the metal is produced in a suitable powder form. Some common material families used in MIM are stainless steel, low alloy steel, tool steel, titanium, copper, tungsten, and hard metals. MIM products' mechanical properties are superior to cast products in most cases and slightly inferior to wrought products. Cast and MIM components both have some microstructural voids as a result of the processing methods. The cast voids are usually large due to the cooling of liquid to solid while the MIM voids are typically fine and well distributed across the microstructure after sintering. The large voids of the cast components result in the inferior properties than MIM components. Even full densification (microstructure without voids) can be attained in MIM by post sintering techniques such as hot isostatic pressing. The dimensional variability of the MIM process is associated with the amount of shrinkage that the component experiences during the debinding and sintering. Components shrink about 1% during the debinding operation and about 15%–25% after sintering.

1.2.2 Design consideration

Before selecting this technology, capital investment, production costs, production rate as well as performance and quality of the part are the factors that need to be taken into consideration. As a general rule of thumb, components that are produced by press and sinter technology (generally less than 100 g) can be easily manufactured by MIM. Typically, an average size of 15 g is common for a MIM component; however, components in the range around 0.030 g are also possible.⁴ Table 1.1 presents the lower and upper specifications of the MIM process.

Table 1.1

MIM process generally produces good surface finish. Typically, the surface finish of 0.8 μm can be achieved easily. However, surface finish as smooth as 0.3 μm is also possible. The surface finish generally depends on the chemistry of powders used and the sintering conditions.

As MIM involves postmolding steps of debinding and sintering, there are some design recommendations that should be considered to get high-quality MIM product, as listed:

1. Avoid components over 12.5 mm thick. In cases where thick sections are desired, special modifications to the binder system should be made to debind thick sections.

2. Avoid components over 100 g in mass.

3. Avoid holes smaller than 0.1 mm in diameter.

4. Avoid wall thinner than 0.1 mm.

5. Maintain uniform wall thickness in order to attain smooth flow during molding.

6. Avoid sharp corners. The desired radius is greater than 0.05 mm.

7. Avoid internal undercuts.

8. Design with a flat surface to aid in sintering.

1.2.3 Powders for metal injection molding

Powders shape, size, and its distribution play a decisive role in determining the overall quality of the MIM product. Metals or alloy powders that can be manufactured to a sufficiently small size (<45 µm), sinterable, and do not possess a melting point lower than the decomposition temperature of the binders can be utilized for MIM. The low-melting temperature and strong surface oxides and their interference with sintering make aluminum and magnesium less desirable for MIM. However, both have been successfully processed by MIM with limited commercial success.⁵ More details on Al and Mg-MIM can be found in Chapter 4, Impurity Management in Reactive Metals Injection Molding, and Chapter 5, Potential Feedstock Compositions for MIM of Reactive Metals, of this book. Common MIM metals and alloys include stainless steel, low-alloy steels, tool steels, copper and its alloys, titanium and its alloys, soft magnetic alloys, refractory metals, and cemented carbides. The ideal MIM powder characteristics are as follows⁶–⁸:

1. Powder particle size (D90) of <22 µm for most of the metals and alloys for good sinterability and surface finish since finer powders sinter more readily than coarser powders. However, for reactive powder such as titanium, this limit can be set at ≤45 µm as the risk of impurity pick-up during sintering also increases with a higher surface area of the fine particles. In addition, for reactive powders (titanium, aluminum, magnesium, and zirconium), the probability of explosion increases simultaneously with decreasing particle size.

2. It is often sought to incorporate a high proportion of metal powder in the feedstock. In other words, powders having a high packing density are desirable. The spherical or near-spherical shape should, therefore, be preferred (see Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry for more details). However, irregular shape powders in the case of titanium and its alloys have also been widely studied as they offer much lower cost (~45 $/kg) compared to spherical powders (~250 $/kg).

3. The powder particles should have high surface purity to maintain uniform interaction with the binder system.

4. The powder particles should be void-free.

Table 1.2 compares the different powder production methods with respect to price, shape, size, and materials that can be processed.

Table 1.2

High-purity argon atomization is the principal technique used to produce reactive metals powders. However, aluminum powders are also produced via air atomization. Other fabrication techniques, such as plasma atomization, are also sometimes used for reactive metals powder production.

1.2.4 Binder selection

The binder system is an integral part of the MIM process. It controls the shaping stage of the MIM process and then holds the powder particles until the initial stage of sintering. By achieving this, a binder system usually has three components: a low molecular weight component that provides the necessary flowability during molding, a backbone polymer that provides the green strength, and a surfactant which acts as a bridge between the binder and powder particles (see Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry for more details). Some common binder systems are listed in Table 1.3.⁹,¹⁰

Table 1.3

Thermoplastic and thermosetting are two common types of polymers. Thermoplastic polymers are formed by repeating small monomer groups along the chain length without cross-linking. On the other hand, in thermosetting polymers, monomers undergo cross-linking, which results in the formation of a three-dimensional rigid structure. The cross-linking of thermosetting polymers upon heating can be helpful during the molding process since it may provide the necessary green strength. However, due to their complicated decomposition processes, thermosetting polymers are rarely used in MIM.

The composition of the binder plays a significant role in determining the binder viscosity and flow behavior, especially for mixtures with large differences in viscosities among the components. Viscosity increases as the molecular weight is increased, so a proper selection of the molecular weight of each binder component is necessary. For titanium MIM, careful selection of binder components is extremely important (discussed more in detail in Chapter 2: Design Strategy of Binder Systems and Feedstock Chemistry) because titanium is extremely reactive to elements such as O, N, and C.

1.2.5 Feedstock preparation

In the next step, the metal or alloy powders are mixed with preselected binders to produce a homogeneous mixture—also known as a feedstock. The mixing operation should be thorough enough to ensure that each particle is coated with the selected binder. Also, homogeneous mixing of feedstock is crucial to the final product quality, as any inhomogeneities such as bubbles, binder pockets, and powder segregation will subsequently be carried over to the injection molding stage. To ensure this, different types of shear mixers are available in the market nowadays. These include twin-screw extruder, double planetary, single screw extruder, plunger extruder, twin-cam extruder, shear roll compounder, and most common sigma or z-blade mixers. Some of the common mixers are shown in Fig. 1.2.

Figure 1.2 Some common mixers/kneaders used in feedstock preparation: (A) a z-blade sigma mixer,¹¹ (B) a shear roller,¹² and (C) torque mixer.¹³ (B) Courtesy Mr. Frank Langer, Bellaform GmbH, Germany; (C) courtesy HAAKE ThermoFisher Scientific.

With the recent advancements in MIM technology and a better understanding of the process, some companies such as ThermoFisher Scientific offer a wide range of feedstock preparation equipment from laboratory-scale simple mixing to industrial-scale compounding and pelletizing, Fig. 1.3.

Figure 1.3 Classic strand pelletizing.¹⁴ Courtesy ThermoFisher Scientific.

1.2.6 Molding operation

The design of injection molding machines irrespective of the suppliers has certain general design features in common, which are necessary to carry out and control the injection molding process. The most important components of the injection molding machine are the injection unit, the clamping unit, and the tooling attached to the clamping unit (Fig. 1.4). These units are generally placed horizontally. In fact, the maximum clamping force is the main characteristic by which the power and size of an injection molding machine are defined. Molding parameters such as injection pressure, injection speed, mold quality, and mold temperature are very important for preparing parts without defects and minimum porosity.

Figure 1.4 A schematic overview of injection molding machine.¹⁵

Barrel temperature is very important; if the temperature is too low, the feedstock may freeze before the mold cavity is completely filled. If the temperature is too high, it will lead to very low viscosity that causes problems such as molten feedstock dripping out of the nozzle, flashing, and prolonged cooling times. It should also be considered that some heat is generated in the barrel by the frictional forces between the screw and the feedstock. Typical melt temperatures for common wax-polymers systems are between 150°C and 190°C, and the mold temperature is 25°C–55°C. Typical melt temperatures for catalytic systems are 200°C–260°C, and the mold temperature is 100°C–150°C.

Owing to the higher thermal conductivity of the feedstock, MIM injection speeds are typically higher than pure polymers. The injection speed is typically set at the minimum injection speed required to completely fill the component's cavity without any defects. Too low an injection speed will result in surface imperfections such as flow lines and incomplete fill. Too high an injection speed will result in a flash due to powder/binder separation and can result in an explosion in the case of reactive powders.

These days even small bench-top injection molders are also available in the market, Fig. 1.5.

Figure 1.5 HAAKE MiniJet pro piston injection molding system by ThermoFisher Scientific. Such small injection molders provide an excellent opportunity to test-run feedstocks on a laboratory scale.

1.2.7 Debinding

After the injection molding step, the binder system becomes a disposable component and hence, must be removed from the samples. During debinding operation, special attention is paid to the shape retention of the molded samples. The method of binder removal has a considerable effect on the molded sample shape retention, uniformity of shrinkage, and final product mechanical properties. Therefore proper selection of the debinding method is important for good quality control, particularly for reactive materials such as titanium, as explained in Chapter 2, Design Strategy of Binder Systems and Feedstock Chemistry. The most commonly applied debinding techniques include thermal debinding, vacuum debinding, catalytic debinding, and solvent-thermal debinding. The solvent debinding, combined with thermal debinding, is the most widely used technique in the MIM industry. It involves two steps: (1) removal of primary binder usually by solvent dissolution, (2) removal of backbone polymer, and surfactant by a thermal process without cracking or degrading the molded part. Some of the important points that must be considered during solvent-thermal debinding are summarized below:

1. The dissolution rate of the primary binder in the solvent increases as the primary binder is liquefied. This implies the temperature of the solvent must be chosen with great care.

2. The reaction of the backbone polymer with the solvent should not cause any part distortion.

3. The solvent should not have high vapor pressure at debinding temperatures. Care must be taken if any of the fire hazard solvents are used.

When the primary binders are removed, no diffusion bonding between powder particles takes place, as the first-step debinding is usually performed at low temperatures. It is the backbone binders and interparticle friction that hold the powder particles together and maintain the shape after the solvent debinding. The secondary binders are removed thermally at moderate-to-high temperatures and called thermal debinding. The thermal debinding is achieved by heating the parts slowly to the temperature where the secondary binder evaporates. At those temperatures, interparticle diffusion is enough to hold the parts together.¹⁶ Table 1.4 presents multiple binder systems with their debinding method, debinding temperature, and approximate debinding rates for regular MIM parts.¹⁷

Table 1.4

*It is worth mentioning here that these debinding rates are just an estimate. Actual debinding rates may greatly vary depending on the powder particle size, debinding temperature, molecular weight of the primary component, and its interactions with metal powder and the other components of the binder